Well, recruiters don’t look at PerlMonks, but most Perl programmers do with some frequency. Being a participant may emphasize that you do (or do not) know what you’re talking about, but really it is your work that speaks for you or against you most clearly. No matter what else you do in a workplace, don’t piss anyone else off. You need a body of work that you can demonstrate, and you need good co-worker references. And none of that is ever going to do your selling for you.
Marketability is strictly determined by your skills of self-promotion; that is to say, marketing. I have a friend who is an excellent sysadmin, but, bless him, he couldn’t promote himself out of a paper bag. Talks about amazing stuff that he routinely does, and it really is amazing stuff ... but put him into an interview and you can almost hear the blam!! as the bullet hits the foot. Again. So, participation in a forum is definitely something that you can use in self-marketing efforts, but your success is largely going to depend on your abilities of self-marketing in general. It’s a skill that must be learned.